All-optical quantising and coding are key functionalities enabling all-optical analogue-to-digital conversion and multilevel-to-binary conversion. An optical analogue to digital converter takes at its input an optical input signal which has a time varying intensity (such as a train of optical pulses of differing intensity levels) with the intensity encoding information and produces at its output an optical digital representation of the input signal, such as a train of digital values corresponding to the input pulse train. This output signal may be encoded in a variety of ways, including binary encoding, Gray scale and two's complement binary. The common link is that each encoding level or “bit” of the output signal is digital and may therefore have only one of two optical states—a low state and a high state. Typically the low state is defined as zero intensity and the high state as some non-zero intensity.
We are aware of a previous attempt at providing an optical analogue to digital converter described in the paper Design Considerations of All-Optical A/D conversion: Non-linear Fiber-Optic Sagnac-Loop Interferometer based Optical Quantizing and coding of Kensuke Ikeda et al, Journal of Lightwave Technology, Vol. 24, No. 7, July 2006. In that paper Quantising and coding is achieved by exploiting the nonlinear optical loop mirrors (NOLMs) characteristic. Specifically the ability of such NOLMS to exhibit more than one transition from a low state to a high state and back to a low state was proposed as the mechanism for providing the required output value for each bit of a 2 bit output signal. However, a problem with the use of NOLMS foreseen by the author of that paper was that the power levels needed at the input for a practical system with more than 2 bits of output are excessive making it unsuitable for many applications, if indeed it could be realised at all.